Core-shell composites for electrodes in metal-ion batteries

ABSTRACT

A battery electrode composition is provided comprising core-shell composites. Each of the composites may comprise a core and a multi-functional shell.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application for patent is a Continuation of U.S. patentapplication Ser. No. 13/776,360, entitled “Core-Shell Composites forSulfur-Based Electrodes in Metal-Ion Batteries,” filed Feb. 25, 2013,which claims priority to Provisional Application No. 61/604,394,entitled “Core-Shell Nanoparticles for Sulfur-Containing Cathodes inMetal-Ion Batteries,” filed on Feb. 28, 2012, both of which areexpressly incorporated by reference herein.

BACKGROUND

Field

The present disclosure relates generally to energy storage devices, andmore particularly to metal-ion battery technology and the like.

Background

Owing in part to their relatively high energy densities, light weight,and potential for long lifetimes, metal-ion batteries are usedextensively in consumer electronics. In fact, lithium-ion (Li-ion)batteries, for example, have essentially replaced nickel-cadmium andnickel-metal-hydride batteries in many applications. Despite theirincreasing commercial prevalence, further development of these batteriesis needed, particularly for applications in low- or zero-emissionhybrid-electrical or fully-electrical vehicles, energy-efficient cargoships and locomotives, aerospace, and power grids. Such high-powerapplications will require electrodes with higher specific capacitiesthan those used in currently existing Li-ion batteries.

Sulfur and sulfur-containing compounds have been investigated as apotential source for higher specific capacity electrodes, in addition tooffering a number of other advantages, including a high theoreticalspecific capacity (1672 mAh/g), high energy density, low voltageoperation, and relative material abundance. Sulfur's specific capacityis the highest among solid cathode compounds known for rechargeableLi-ion batteries and an order of magnitude greater than currentlyavailable commercial cathodes. Its ultra-high specific capacity canenable exceptional gravimetric and volumetric energy densities inrechargeable batteries (e.g., 2600 Wh/kg and 2800 Wh/l, respectively),which is around 4-10 times higher than that of current state of the artLi-ion batteries. Electrochemical reactions in Li/S cells occur atrelatively low voltage (e.g., approximately 30-40% lower than thatobserved in conventional cathodes), allowing greater flexibility indesigning electronic components and minimizing safety risks associatedwith high voltage cathodes. Sulfur is also found abundantly in nature,low cost, and light weight, in addition to having a relatively lowtoxicity.

For all of these reasons, sulfur-based cathodes are being investigatedas a cost-effective, environmentally friendly, performance enhancingcomponent of metal-ion batteries. However, realization of the fullpotential of sulfur-based cathodes in metal-ion batteries has beenhindered by a number of significant challenges, including low electricalconductivity, low ionic conductivity, and the physical instability ofconventional sulfur-based cathodes. Sulfur and sulfur-containingcompounds are highly electrically insulating. The ionic conductivity oflithium in sulfur and sulfur-compounds is also very small, whichtypically slows down the overall rate of the electrochemical reactionsand leads to low power characteristics in Li/S cells. In addition,sulfur cathodes generate intermediate electrochemical reaction products(polysulfides, such as Li₂S_(n)) that are highly soluble in conventionalorganic electrolytes. This leads to sulfur cathode dissolution andre-deposition of electrically-insulating precipitates on the anodesurface, preventing full reversibility of the electrochemical reaction.

Thus, despite the theoretical advantages of sulfur-based cathodes,practical application in metal-ion batteries is difficult to achieve.Several approaches have been developed to overcome these difficulties,but none have been fully successful in overcoming all of them. Forexample, some conventional designs have attempted to address the lowelectrical conductivity by using a conductive carbon additive to formC—S composites, but this does not address the ionic conductivity orcathode instability. Other conventional designs have attempted toaddress the ionic conductivity by using special electrolytes that causethe sulfur to swell, but this often increases the rate of sulfurdissolution. Still other conventional designs have attempted to improveelectrochemical reversibility by eliminating or preventing polysulfideanion precipitation on the anode surface (e.g., via electrolyteadditives to dissolve insulating sulfur-containing precipitates), butthis does not address the more critical problem of sulfur cathodedissolution, or low electrical and ionic conductivity.

One of the more advanced approaches for sulfur cathode stabilizationinvolves the formation of porous S—C composites by forming a porouscarbon matrix and partially filling it with sulfur via melt or solutioninfiltration. This approach, however, still suffers from low volumetriccapacity of the produced composites and still has an unsatisfactorilyhigh cathode dissolution rate.

Accordingly, conventional approaches to address sulfur-based cathodeshortcomings have found only limited success. There remains a need forbetter ways to address the low electrical and ionic conductivity as wellas physical instability of sulfur-based cathodes in metal-ion batteries.

SUMMARY

Embodiments disclosed herein address the above stated needs by providingimproved battery components, improved batteries made therefrom, andmethods of making and using the same.

A battery cathode composition is provided comprising core-shellcomposites. Each of the composites may comprise a sulfur-based core anda multi-functional shell. The sulfur-based core is provided toelectrochemically react with metal ions during battery operation tostore the metal ions in the form of a corresponding metal-sulfide duringdischarging of the battery and to release the metal ions from thecorresponding metal-sulfide during charging of the battery. Themulti-functional shell at least partially encases the sulfur-based coreand is formed from a material that is (i) substantially permeable to themetal ions of the corresponding metal-sulfide and (ii) substantiallyimpermeable to electrolyte solvent molecules and metal polysulfides.

In some designs, at least a portion of the composites may be formed witha substantially spherical, particle morphology of the core and shell. Inother designs, at least a portion of the composites may be formed with asubstantially planar, flake morphology of the core and shell. At least aportion of the substantially planar, flake morphology composites may beaggregated together into a three-dimensional agglomeration, for example.In addition, at least a portion of the composites may be formed withexternal open channel pores, external to the shell in relation to thecore, that extend from an outer edge of the composite towards the core.The open channel pores provide channels for faster diffusion of themetal ions from outside of the composite into the core by reducing theaverage diffusion distance of the metal ions within the composite.

In some designs, at least a portion of the composites may haveelectrically interconnected sulfur-based cores. That is, at least onelayer of their corresponding shells may only partially encase theinterconnected sulfur-based cores, rather than fully encase them. Inthis way, the electrical interconnectivity may be maintained between theindividual sulfur-based cores while the at least one layer substantiallyconformally covers a majority of the interconnected cores.

In some designs, the sulfur-based core may comprise pores mixed in withthe sulfur material. In other designs, the sulfur-based core maycomprise a dense metal-sulfide, which may in some cases compriseindividual nano-sized or micro-sized grains of metal-sulfide linkedtogether with a metal-ion-conductive material. The sulfur-based core mayalso comprise conductive carbon provided to enhance electricalconductivity. The conductive carbon may be provided, for example, in theform of carbon nanoflake(s), graphene segment(s), multi-layered graphenesegment(s), graphite ribbon(s), carbon nanotube(s), nanostructureddendritic carbon, nanoporous carbon, carbon black particle(s), carbononion particle(s), fullerene(s), and/or carbon nanofiber(s).

The material from which the multi-functional shell is formed maycomprise at least one material selected from the group consisting of (i)a metal-ion-conductive ceramic coating, (ii) a polymer coating, (iii) anelectrically-conductive carbon coating, and (iv) a semiconductorcoating. In some designs, the material from which the multi-functionalshell is formed may comprise a composite coating of two or more of thesematerials. The composite coating materials may be arranged in aninterpenetrating configuration such that at least two of the compositecoating materials contact the sulfur-based core, or alternatively, in alayered configuration such that at least one of the composite coatingmaterials does not contact the sulfur-based core. In addition, thematerial from which the multi-functional shell is formed may comprise atleast one material selected to electrochemically react with the metalions during battery operation to store the metal ions in the shellduring discharging of the battery and to release the metal ions from theshell during charging of the battery.

A battery cathode composition of the type described herein may beincorporated into a battery, with the material from which the shell isformed remaining (i) substantially permeable to the metal ions of thecorresponding metal-sulfide and (ii) substantially impermeable toelectrolyte solvent molecules and metal polysulfides during batteryoperation (e.g., over a substantial number of cycles).

A method of producing a battery cathode composition is also provided.The method may comprise, for example, forming a sulfur-based core and atleast partially encasing the sulfur-based core with a multi-functionalshell to form core-shell composites. The sulfur core is provided toelectrochemically react with metal ions during battery operation,whereby the electrochemical reaction stores the metal ions in the formof a corresponding metal-sulfide during discharging of the battery andreleases the metal ions from the corresponding metal-sulfide duringcharging of the battery. The shell may be formed from a material that is(i) substantially permeable to the metal ions of the correspondingmetal-sulfide and (ii) substantially impermeable to electrolyte solventmolecules and metal polysulfides.

According to various embodiments, certain methods may further comprisedispersing core-shell composites in a binder solution, applying thedispersion of core-shell composites to a conductive metal substrate, andair or vacuum drying the metal substrate to yield a substantiallyuniform coating of core-shell composites bonded with a binder on themetal substrate. Other methods may further comprise dispersing coreparticles in a binder solution, applying the dispersion of corecomposites to a conductive metal substrate, air or vacuum drying themetal substrate to yield a substantially uniform coating of coreparticles bonded with a binder on the metal substrate, and depositingthe multi-functional shell in a substantially conformal coating to atleast partially encase the sulfur-based cores and form the core-shellcomposites. The at least partially encasing may comprise depositing themulti-functional shell by a vapor deposition method, such as chemicalvapor deposition, atomic layer deposition, or thermal decomposition ofprecursor molecules in a gas phase.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are presented to aid in the description ofembodiments of the invention and are provided solely for illustration ofthe embodiments and not limitation thereof.

FIG. 1 illustrates an example battery cathode material compositioncomprising core-shell composites according to one embodiment.

FIG. 2 illustrates metal-ion insertion and extraction from a core-shellcomposite during battery operation.

FIG. 3 illustrates several examples of a core-shell internal structurewith and without conductive additives.

FIG. 4 illustrates several examples of large composites with openchannel pores.

FIG. 5 illustrates an example multi-functional shell structure composedof multiple layers.

FIG. 6 illustrates an example multi-functional shell structure composedof multiple interpenetrating materials.

FIG. 7 illustrates an example electrode design with interconnectedsulfur-based cores.

FIGS. 8A-8B illustrate several example designs in which the sulfur-basedcore is composed of a dense metal-sulfide, both with and withoutconductive additives, and with and without external open channel pores.

FIG. 9 illustrates designs in which the composites are formed with asubstantially planar, flake morphology of the core and shell.

FIG. 10 illustrates an example three-dimensional agglomerate structureformed from substantially planar, flake morphology composites.

FIG. 11 is a flow diagram of an example method of producing a batterycathode composition according to various embodiments herein.

FIG. 12 shows example electrochemical performance data of an electrodeproduced according an example embodiment as compared to performance of aconventional electrode.

FIG. 13 illustrates an example Li-ion battery in which the abovedevices, methods, and other techniques, or combinations thereof, may beapplied according to various embodiments.

DETAILED DESCRIPTION

Aspects of the present invention are disclosed in the followingdescription and related drawings directed to specific embodiments of theinvention. The term “embodiments of the invention” does not require thatall embodiments of the invention include the discussed feature,advantage, process, or mode of operation, and alternate embodiments maybe devised without departing from the scope of the invention.Additionally, well-known elements of the invention may not be describedin detail or may be omitted so as not to obscure other, more relevantdetails.

As discussed in the background above, there remains a need in the artfor better addressing the low electrical and ionic conductivity as wellas physical instability of sulfur-based cathodes in metal-ion batteries.The present disclosure accordingly provides or otherwise facilitates thefabrication and use of improved composite materials comprising sulfur orsulfur-based compounds for metal-ion battery cathodes, improvedmetal-ion batteries made therefrom, and methods of making and using suchcomponents and devices. In this way, a more full realization of thepositive attributes of sulfur electrochemistry in metal-ion batteries,improved development of advanced sulfur cathodes, and improveddevelopment of advanced metal-ion batteries may be achieved.

FIG. 1 illustrates an example battery cathode material compositioncomprising core-shell composites according to one embodiment. As shown,each of the composites 102 comprises a sulfur-based core 104 and amulti-functional shell 106 that at least partially encases thesulfur-based core 104, forming a so-called “core-shell” structure. Thesulfur-based core 104 is provided to electrochemically react with metalions during battery operation to store the metal ions in the form of acorresponding metal-sulfide during discharging of the battery and torelease the metal ions from the corresponding metal-sulfide duringcharging of the battery. The multi-functional shell 106 is formed from amaterial that is (i) substantially permeable to the metal ions of thecorresponding metal-sulfide, such as Li, Na, Mg, Al, K, or Ca, and (ii)substantially impermeable to electrolyte solvent molecules and metalpolysulfides.

In some designs, the material or a portion thereof from which themulti-functional shell 106 is formed may also be selected to beelectrically conductive. In some further designs, the material may beselected to additionally be able to store the metal ions of thecorresponding metal-sulfide during discharging (or more generally,during metal-ion insertion) and release them during charging (or moregenerally, during metal-ion extraction), while remaining impermeable tothe solvent molecules and the metal polysulfides. In this case, themulti-functional shell 106 not only protects the sulfur-based core 104from dissolution, but also stores metal ions itself, thereby increasingthe total amount of metal ions that can be stored in an electrode duringbattery operation.

Suitable degrees of permeability and impermeability may be assessed, forexample, based on relative average diffusion coefficients(diffusivities). For example, it may be desirable that the averagediffusivity D(M) of the metal ions through the multi-functional shell106 be larger than the average diffusivity D(solv) of the electrolytesolvent molecules by a factor 100, a factor of 1000, or even a factor of10,000, according to various designs. Similarly, it may be desirablethat that the average diffusivity D(M) of the metal ions through themulti-functional shell 106 be larger than the average diffusivity D(MPS)of the corresponding metal polysulfides by a factor 100, a factor of1000, or even a factor of 10,000, according to various designs.

A cathode may be formed to take advantage of such core-shell structuresby coating a metal substrate 108 (e.g., aluminum foil) with thecomposites 102 and a polymer binder (not shown) from a suspension ofsuch particles in a binder solution. Air or vacuum drying may be used toyield a uniform, dry coating of the polymer-bonded core-shell composites102 on the surface of the metal substrate 108. Alternatively, thecomposites 102 can be mixed or decorated or coated with a polymer binderand deposited on the metal substrate 108 without any solvents using adry powder coating system. In some cases, the coated powder can becompressed (calendared) to achieve higher packing density. In somecases, the coated powder may be exposed to UV light or heated to inducecross-linking of the polymer. The substrate 108 can then be rolled andincorporated as a cathode in an energy storage device, such as a primaryor secondary metal-ion battery (e.g., Li-ion, Na-ion, Mg-ion, Al-ion,K-ion, or Ca-ion batteries).

As explained in more detail below, this new sulfur-based cathode designhas the ability to substantially alleviate the problems of sulfurcathode dissolution, low ionic conductivity, and low electricalconductivity associated with conventional sulfur-based cathodes inmetal-ion batteries. Additional improvements in performance andstability may also be achieved by using polymeric binders capable ofmetal-ion transport but also impermeable to electrolyte solvents.

FIG. 2 illustrates metal-ion insertion and extraction from a core-shellcomposite during battery operation. In this design, the sulfur-basedcore 104 is composed of porous sulfur nanoparticles having a network ofpores 202. This “porous” design allows the sulfur-based core 104 toaccommodate metal-ion (e.g., Li-ion) insertion/extraction while allowingthe overall particle size to remain constant and the multi-functionalshell 106 to remain intact. Maintaining particle size during batteryoperation helps minimize the sulfur-based cathode degradation problem.

The pores 202 can be interconnected or isolated, as is furtherillustrated in FIG. 2. In addition, the composite's nano-scale size(e.g., about 20-2,000 nm in diameter) reduces the metal diffusiondistance, while its near-spherical shape enables a high electrodepacking density, electrode uniformity, and minimal surface area (perunit volume for a given particle size) to reduce undesirable sidereactions. These combined measures offer high cathode capacity, a highdegree of active material utilization and material loss prevention, andfast charge/discharge kinetics for the battery.

In order to enhance the electrical conductivity within individualcomposites and in order to enhance the long-term stability of the core,particularly for composites larger than 50 nm, a conductive additive(e.g., a conductive carbon of nano-sized dimensions, preferably formingan electrically interconnected carbon network) can be additionallyincorporated in the sulfur-based core.

FIG. 3 illustrates several examples of a core-shell internal structurewith and without conductive additives. As shown, the conductiveadditives can be incorporated, for example, as separate nanostructureswithin the core, such as smaller carbon nanoflakes, graphene segments,short carbon nanotubes, segments or particles of nanoporous carbon(e.g., with pores in the range of about 0.3 to 10 nm), small carbonblack particles, carbon onions, fullerenes, dendritic carbon, porouscarbon, small carbon nanofibers, etc., or as a thin coating (e.g.,disordered but electrically conductive carbon) around individual sulfurnanoparticles comprising the porous sulfur core. In particular,structure 310 illustrates a porous sulfur-based core 104 without anyconductive additives. Structure 320 illustrates a porous sulfur core 104with a conductive carbon additive provided as separate nanostructures302 from the internal nanoparticles. Structure 330 illustrates a poroussulfur core 104 with a conductive additive provided as a coating 304around individual sulfur nanoparticles. Alternatively, the carbonadditive in the core may be in the form of a single porous particle withinterconnected pores at least partially filled with sulfur.

The carbon additives in the core additionally provide structuralreinforcement of the composite structure and assist in maintainingconstant dimensions during metal ion transport to/from the core from/tothe electrolyte. In some designs, particles or nanostructures other thancarbon can provide structural reinforcement of the composite structureand assist in maintaining the constant dimensions during electrodefabrication and/or during metal ion transport to/from the core from/tothe electrolyte during the battery operation.

In some applications, it may be advantageous to use relatively largediameter composites 102 (e.g., about 300 nm to about 20 μm). Largersized composites are easier to handle. However, the metal ion diffusiondistances become large. Since the average diffusion time is proportionalto a square of the diffusion distance, larger diffusion distances leadto slower metal ion insertion and extraction, and ultimately to poorerpower performance of the cathode employing such composites. To enhancethe ionic conductivity and power performance of larger composites, insome designs, so-called open or external “channel” pores may beintroduced.

FIG. 4 illustrates several examples of large composites with openchannel pores. As shown, open pores 402 may be introduced into thecomposite structure to provide ion channels with reduced averagediffusion distances from the electrolyte to the center of the core 104.The open channel pores 402 may be introduced into any of the compositestructure variants herein. Structure 410, for example, illustrates acomposite 102 having internal (closed for electrolyte solvent access)pores 202 in addition to the external open pores 402, while structure420, as another example, illustrates a composite 102 having internalclosed pores 202 and conductive additives 302 in addition to theexternal open pores 402. As is further illustrated, the dimensions ofthe external open channel pores 402 may vary, generally, for example,from about 0.5-100 nm, or more preferably, from about 1-10 nm.Furthermore, it may be advantageous that the open pores 402 occupy lessthan about 35% of the overall composite volume, or even less than about15% of the overall composite volume. As shown, the walls of the openpores 402 may be coated with various multifunctional shells 106, asdesired.

Turning to the composition of the multi-functional shell 106, themulti-functional shell can be made of, for example, (1)metal-ion-conductive ceramics (e.g., oxide based, oxy-fluoride based,fluoride based, phosphate based, iodide based, or different materialscomprising a metal oxide or fluoride or phosphate or iodide), (2)metal-ion-conductive polymers, (3) a metal-ion-conductive,electrically-conductive carbon (e.g., sp²-bonded carbon), or (4) asemiconductor material such as silicon. It is important that the shellremains largely impermeable to the electrolyte solvent during batteryoperation (i.e., during battery charge, storage, and discharge) andtherefore does not produce defects or pores during battery operation. Insome designs, the material from which the multi-functional shell 106 isformed may comprise a composite coating of two or more of the abovematerials, in a number of different arrangements and configurations. Insome designs, the multi-functional shell 106 can be a composite of twoor more structurally and/or chemically different materials, in which atleast one material is permeable by metal ions and forms continuouschannels within the shell through which ion transport may take place.

FIG. 5 illustrates an example multi-functional shell structure composedof multiple layers. Each of the layers 502, 504 may have its owncomposition. For example, in one design, the shell may be composed of afirst layer 502 made of metal-ion-conductive ceramics, surrounded by asecond layer 504 of electrically conductive carbon. In another design,the shell may be composed of a first layer 502 made of carbon surroundedby a second layer 504 of an ionically conductive polymer (e.g.,conductive of Li+ ions in the case of a Li or Li-ion battery). In stillother designs, the shell can comprise various layered combinations ofmetal-ion-conductive ceramics, metal-ion-conductive polymers, carbon,and/or semiconductors.

In the illustrated design, the composite coating materials are arrangedin a layered configuration such that at least one of the compositecoating materials does not contact the sulfur-based core. However, inother multi-material, composite coating designs, the composite coatingmaterials may be arranged in an interpenetrating configuration such thatat least two of the composite coating materials contact the sulfur-basedcore. In some designs, it has been found advantageous for themulti-functional shell 106 to contact 20% or more of the core 104external surface, and even more advantageous for the multi-functionalshell 106 to contact 60% or more of the core 104 external surface.

FIG. 6 illustrates an example multi-functional shell structure composedof multiple interpenetrating materials. In particular, a partialcross-section of an example composite structure 610 of this type isshown with a multi-functional shell composed of a porous skeleton 602and a pore filler 604 interpenetrating the porous skeleton 602. Theporous skeleton 602 may be configured to provide structural integrity,while the pore filler 604 may fill the pores of the skeleton 602 in sucha way so as to suppress the transport of, for example, polysulfidesthough the multi-functional shell. In some designs, the porous skeleton602 may comprise electrically conductive carbon, for example, with itspores being completely filled with another material selected for thepore filler 604 that is impermeable to, e.g., lithium sulfides. Thisinterpenetrating structure is particularly attractive when one of thematerials has a significantly higher electrical conductivity than theother material, such that the total electrical resistance of themulti-functional shell may be minimized. Similarly, the interpenetratingstructure is also particularly attractive when one material has asignificantly higher ionic conductivity than the other material, suchthat the total ionic resistance of the multi-functional shell may beminimized.

The filler material 604 may comprise, for example, metal-ion-conductiveceramics (e.g., oxide based, oxy-fluoride based, fluoride based,phosphate based, iodide based, or different metals comprising metaloxide or fluoride or phosphate or iodide) or metal-ion-conductivepolymers. In some cases, the filler material need not be ionicallyconductive and the ion transport may take place through the skeletonmaterial. While this lack of ionic conductivity of either the filler orskeleton material may be disadvantageous in some applications requiringmore rapid ion transport (e.g., high-power batteries), the overall ionicconductivity through the composite shell may be sufficient for otherapplications (e.g., low- or moderate-power batteries).

One advantage of using a conductive porous skeleton within themulti-functional shell is that the shell may remain electricallyconductive despite containing an electrically isolative filler material.Another advantage is better mechanical integrity. Another advantage isthe additional flexibility during synthesis of the core-shellsulfur-based composites. For example, in this way the sulfur-based corematerial can first be infused inside the porous skeleton shell and thenthe pores in the skeleton can be filled with a metal sulfide-impermeablefiller material.

Returning to FIG. 1, in some applications, it may be advantageous todeposit or produce the multi-functional shell 106 (or at least one ofthe components thereof) after depositing the sulfur-based cores 104 onthe surface of the metal substrate 108. In this way, at least a portionof the composites 102 can be made to have interconnected sulfur-basedcores 104, with their corresponding shells 106 only partially encasingthe interconnected sulfur-based cores 104.

FIG. 7 illustrates an example electrode design with interconnectedsulfur-based cores. In this design, because the sulfur-based cores 104are electrically connected when deposited on the surface of the metalsubstrate 108, the subsequent deposition of an electrically insulatingmaterial as the shell 106 or an electrically insulating component of theshell 106 does not reduce the electrical connectivity between theindividual composites 102. Good electrical connectivity of thecomposites within the electrode is important for their application inhigh-power batteries, for example.

In addition to the porous designs discussed above, the sulfur-based coremay alternatively be composed, for example, of a fully encapsulated,densely packed metal-sulfide (M_(x)S). Denser metal-sulfide cores may beadvantageous in certain applications for a number of reasons. They havehigher density, which leads to higher volumetric capacity and energydensity. The pores shown in FIGS. 2-4, for example, may be ofinsufficient volume, which may lead to the breakage of the composites orthe multi-functional shell during metal ion insertion. Alternatively,the pores may be of too high volume, which will reduce the volumetricperformance of the composites. Metal sulfides have better mechanicalstrength than porous sulfur, which is important during electrodefabrication since the breakage of the core-containing particles ishighly undesired and may lead to inferior properties and inferiorstability of the battery electrodes. Metal sulfides additionally exhibithigher temperature stability than sulfur. Similar to sulfur, metalsulfides provide for the formation of composites that are stable inorganic or ionic liquid (IL) electrolytes. But in contrast to sulfur,metal sulfides can be used together with metal-free anodes, such asLi-free graphite anodes for Li batteries or anodes comprising materialswhich form electrochemical alloys with Li (e.g., Si, Sn, Ge, Mg, andothers) for Li batteries. Composite volume changes may accordingly bereduced during battery operation (i.e., metal-ion insertion/extraction),since the composites begin with metal incorporated into the sulfur-basedcore. Since the formation of pores is no longer needed within the core,these designs are also simpler to manufacture.

The use of metal-sulfides allows production of high quality,structurally stable, more precisely tuned multi-functional shells athigher temperatures. For comparison, the melting point for sulfur isapproximately 113° C. while the melting point for Li₂S is approximately938° C. High temperature chemical vapor deposition (CVD), sol-gel, andannealing techniques, for example, can be used to optimize ion transportthrough the shell, electrical conductivity of the shell, and otherphysical, chemical, or mechanical properties of the shell.

FIGS. 8A-8B illustrate several example designs in which the sulfur-basedcore is composed of a dense metal-sulfide, both with and withoutconductive additives, and with and without external open channel pores.In particular, structure 810 illustrates a dense metal-sulfide as thecore 104 without any conductive additives. Structure 820 illustrates ametal-sulfide as the core 104 with a conductive carbon additive providedas separate nanostructures 302 from the sulfur nanoparticles 102,similar to structure 320 discussed above with reference to FIG. 3. Asdiscussed above, such conductive additives can be incorporated, forexample, as separate nanostructures within the core in a variety ofways, such as using smaller carbon nanoflakes, graphene segments, shortcarbon nanotubes, carbon onions, fullerene, multi-layered graphenesegments, graphite ribbons, small carbon black particles, small carbonnanofibers, etc.

In some designs it may be advantageous to have electricallyinterconnected conductive carbon additives to enhance the electrontransport rate within the core. In structure 830, the metal-sulfide corecomprises individual nano-sized or mico-sized grains 804 ofmetal-sulfide linked together with a metal-ion-conductive material 806.The metal-ion-conductive material 806 may also be electricallyconductive in some designs.

The structure 840 is of a larger diameter design (e.g., with a diameterin the range of about 200 nm to 20 μm) and contains open channel pores402 (e.g., sized in the range of about 0.5 to 100 nm, preferably in therange of about 1 to 10 nm). As discussed above, the open channel pores402 reduce the average ion diffusion distance. The structure 850 issimilar to the structure 840, but additionally contains conductiveadditives 302 within the metal sulfide core. It will be appreciated thatthe metal-sulfide core designs can be formed with any of the shellstructures or core interconnectivities described herein, as desired.

In still other designs, the carbon additive in the core may be in theform of a single porous particle with interconnected pores at leastpartially filled with sulfur. As discussed above, the carbon additivesin the core additionally provide structural reinforcement of thecomposite structure and assist in maintaining constant dimensions duringmetal ion transport to/from the core from/to the electrolyte. In somedesigns, particles or nanostructures other than carbon can providestructural reinforcement of the composite structure and assist inmaintaining the constant dimensions during electrode fabrication and/orduring metal ion transport to/from the core from/to the electrolyteduring the battery operation.

According to various embodiments, the core-shell composites can befabricated in different architectural forms, including, for example, acore-shell powder in which the composites are near-spherical orcore-shell flakes or nanoflakes (or aggregates thereof) in which thecomposites have a more planar morphology. FIG. 1 shows one design inwhich the composites 102, in the illustrated example, are formed with asubstantially spherical, nanopowder morphology of the core and shell.However, other designs and morphologies may be advantageous in certainapplications.

FIG. 9 illustrates other designs in which the composites are formed witha substantially planar, flake morphology of the core and shell. Asshown, core-shell flakes with both porous sulfur and dense metal-sulfidecores can be formed in this way. In particular, structure 910illustrates a porous sulfur material as the core 104 while structure 920illustrates a dense metal-sulfide material as the core 104. In eithercase, the sulfur core 104 may be constructed around a 2D backbone 902(e.g., multi-layer graphene) to provide the substantially planar, flakemorphology. As discussed above, the porous sulfur or metal-sulfide coreaccommodates metal insertion/extraction, allowing overall composite sizeto remain constant and the multi-functional shell 106 to remain intact.The further illustrated structure 930 shows an example sulfide core 104with a conductive additive 302. As with the nanopowder designs above,conductive additives can be added to the core in the form of smallercarbon nanoflakes, graphene segments, short carbon nanotubes, smallcarbon black particles, small carbon nanofibers, activated carbon,microporous carbon, mesoporous carbon, etc. It will be furtherappreciated that the flake designs can be formed with any of the shellstructures or core interconnectivities described herein, as desired.

The planar morphology provides several key advantages in terms of powercapabilities. Planar particles offer higher electrical conductivity dueto larger area contacts and the ability to propagate all the way fromthe current collector to the surface of the electrode. The highersurface area (per unit volume) of the planar particles reduces thethickness of the sulfur-based core, which helps to mitigate sulfur's lowelectrical and ionic conductivities. Particle swelling during metalinsertion/extraction due to insufficient porosity and other defects ofthe sulfur-containing core can also be accommodated. The presence of theplanar backbone 902 helps preserve the integrity of the top and bottomsurface area of the multi-functional shell 106, limits shell damage tothe small fraction of surface area at the edges of the flake particle,and thus minimizes exposure to electrolyte solvent.

FIG. 10 illustrates an example three-dimensional agglomerate structureformed from substantially planar, flake morphology composites. In thisexample, the three-dimensional agglomerate structure 1010 includes aplurality of flake composites 1002 with pores or voids 1004therebetween, and is shaped as a generally spherical granule. It will beappreciated, however, that other shapes may be created as desired for aparticular application, such as an ellipse, an ellipsoid, a rod, orother shapes. It will further be appreciated that while terms like“spherical” and “ellipsoidal,” for example, are used to describe theshape of such a three-dimensional structure, these terms are notintended to convey or in any way require that the agglomerates take on amathematically precise geometric figure. These terms are only used toconvey the general shape for illustration purposes. It will beappreciated that, in practice, the three-dimensional agglomeratestructure 1010, for example, may simply be substantially round, and notprecisely spherical or even ellipsoidal.

Various methods can be used to form the sulfur-based core and themulti-functional shell, examples of which are described in detail below.In these examples, it will be appreciated that care should be taken toensure compatibility between the particles to be coated and theprotective coating, as well as the deposition process used (i.e., thecoating synthesis conditions should not destroy the particles due totemperature, chemical dissolution, etc.).

FIG. 11 is a flow diagram of an example method of producing a batterycathode composition according to various embodiments herein. As shown asulfur-based core is formed for electrochemically reacting with metalions during battery operation (block 1110). As discussed above, theelectrochemical reaction stores the metal ions in the form of acorresponding metal-sulfide during discharging of the battery andreleases the metal ions from the corresponding metal-sulfide duringcharging of the battery. The sulfur-based core is then at leastpartially encased with a multi-functional shell to form core-shellcomposites (block 1120). The shell is formed from a material that is (i)substantially permeable to the metal ions of the correspondingmetal-sulfide and (ii) substantially impermeable to electrolyte solventmolecules and metal polysulfides.

At the appropriate time, the resultant battery cathode composition maybe formed into a cathode. This may be achieved, for example, bydispersing the core-shell composites in a solvent mixed with a dissolvedpolymer binder and optional conductive carbon additives (optional block1130). The suspension of core-shell composites in the polymer bindersolution may then be applied to a conductive metal substrate (optionalblock 1140). Subsequently, air or vacuum drying the metal substrate maybe used to yield a substantially uniform coating of core-shellcomposites on the metal substrate, bonded (e.g., glued) together with apolymer (optional block 1150), thereby creating a battery cathode.Alternatively, the blocks 1130, 1140, and 1150 may be replaced by a dryparticle deposition method to yield a substantially uniform coating ofcore-shell composites mixed with optional conductive additives andbonded with a polymer binder on the metal substrate. Finally, thepacking density of the produced electrode can be increased by a processcalled calendaring or pressure-rolling (optional block 1160).

The above steps, procedures, and techniques may be practiced andsupplemented in a variety of ways to achieve the various structuresdescribed herein.

In a first example, a method of forming a porous sulfur core, with orwithout conductive additives, using a disproportionation of thiosulfatesolution will be described. Here, the synthesis of sulfur powder andnanopowder suspensions may be achieved by disproportionation of awater-based sodium thiosulfate solution (Na₂S₂O₃), as is known in theart. According to this process, concentrated HCl is added to a watersolution of Na₂S₂O₃ to produce a yellow suspension of sulfur particles(sulfur sol). Other precursors can be added to the stable suspension ofsulfur nanopowder to later induce uniform metal oxide or carbon coatingson their surface.

In order to produce sulfur nanoparticles of the desired morphology andsize as described herein (including a porous sulfur core in thisexample), the growth of the primary sulfur nanoparticles may berestricted (e.g., to below about 5-10 nm) and their agglomerationinduced to form porous granules of about 20-600 nm in diameter (forspherical particles), or to form thin layers of less than about 2-200 nmin thickness on each side of a planar backbone substrate (for planarparticles). In the case of planar particle geometry, sulfurnanoparticles or a continuous sulfur layer may be nucleated on thesurface of the planar backbone (e.g., functionalized graphene), orsulfur nanoparticles may be attached to the backbone surface. In thelatter case, the carbon backbone and the sulfur nanoparticles shouldhave opposite charges on their surface. The desired size and porosity ofthe sulfur-containing core can be optimally tuned by adjusting theconcentration of the thiosulfate solution, adding surfactants, adjustingpH of the solution and using acids other than HCl, or varying thetemperature during synthesis.

Introduction of functionalized conductive additives (e.g., carbon) intothe solution allows formation of a porous composite composed of sulfurand conductive additives. Incorporation of organics (e.g.,polysaccharides) into the solution during wet chemistry synthesis ofsulfur nanoparticles creates a carbon-containing organic layer, whichcan then be transformed into a conductive carbon layer on the outersurface of the entire sulfur core or on the surface of individual sulfurnanoparticles making up the porous sulfur core.

Surfactants can be used to create a desired electrical charge on thesurface of individual sulfur nanoparticles or agglomerates to facilitatecoating with other electrically charged materials. For example,surfactants can be used to induce a positive charge on the surface ofsulfur nanoparticles or agglomerates intended to be additionally coatedwith a polymer having negatively charged surface functional groups(e.g., polysaccharides). Subsequent exposure of the mix to elevatedtemperature, elevated pressure, or to a more acidic environment leads topartial decomposition of the polymer layer, forming a conductive carboncoating on the sulfur surface.

In a second example, a method of forming a porous sulfur core, with orwithout conductive additives, via “miniemulsion” will be described.Miniemulsion refers to nano-sized (e.g., about 20-600 nm) emulsion,prepared by emulsification of two immiscible liquids in the presence ofsurfactants and a third agent with low solubility in one of the liquids(often called a hydrophobe). For the preparation of sulfurnanoparticles, sulfur is converted to the liquid phase by mixing thesulfur with water, surfactant, and hydrophobe, and heating the mixturein a pressurized reactor to above the sulfur-melting temperature. Theliquid phase sulfur may then be incorporated into a miniemulsion withwater using ultrasound or high rate mixing. Once a stable emulsion isachieved, the mixture is cooled down to ambient temperature, yielding asuspension of sulfur nanoparticles in water. Other conductivenanoparticles can be added to the miniemulsion to form sulfurnanocomposites. Subsequently, the porous sulfur core may be formed fromthe sulfur nanoparticles and nanocomposites using controlledflocculation.

In a third example, a method of forming a dense metal-sulfide core, withor without conductive additives, via wet chemistry techniques will bedescribed. For simplicity, the following description relates to thesynthesis of lithium sulfide-based structures (i.e., when M=Li).However, it will be appreciated that when M=Na, Mg, Al, K, or Ca,similar approaches can be utilized.

To prepare metal-sulfide nanoparticles or other nanostructures using awet chemistry approach, the metal-sulfide is first dissolved in water,ethanol, or another appropriate solvent. A non-solvent for Li₂S that ismiscible with water (e.g., acetone, etc.) is added to precipitate Li₂Sfrom the solution. After precipitation, isolation of Li₂S nanoparticlesmay be achieved by centrifugation of the suspension and vacuum drying ofthe solvent residuals.

To prepare Li₂S nanoparticles with conductive additives, the desiredadditives can be suspended in the Li₂S solution. Upon non-solventaddition, suspended conductive additives serve as nucleation centers forLi₂S precipitation, yielding nanoparticles consisting of metal-sulfidewith conductive additives enclosed inside.

The size, size distribution, and morphology of precipitated particlescan be varied by the non-solvent used, rate of non-solvent addition,application of shear forces to the solution via mechanical mixing orultrasound, addition of surfactants, and concentration of the initialLi₂S in a water solution.

In a fourth example, a method of forming a dense metal-sulfide core,with or without conductive additives, via gas phase synthesis will bedescribed. For simplicity, the following description relates to thesynthesis of lithium sulfide-based structures (i.e., when M=Li).However, it will be appreciated that when M=Na, Mg, Al, K, or Ca,similar approaches can be utilized.

To prepare Li₂S nanoparticles using gas phase synthesis, a watersolution of Li₂S (with or without conductive additives) may bespray-dried in air. The size, size distribution, and morphology of thefinal nanoparticles can be controlled by varying the degree of solutionatomization during spray drying, the drying temperature, and therelative concentration of components in the solution.

Synthesis of dendritic Li₂S nanoparticles can be achieved by firstemulsifying a water solution of Li₂S in oil phase (e.g., hexane,toluene, etc.), with surfactants (non-ionic, cationic, or anionic) addedfor stabilization. The emulsion is then diffused through oil phase toevaporate the water and form dendrite Li₂S nanoparticles. Size andmorphology of the nanoparticles can be controlled by varying the initialconcentration of the Li₂S solution, the nature and amount of surfactant,the relative ratio of water and oil in the mixture, and rate of waterevaporation from the emulsion.

In a fifth example, a method of forming a metal oxide protective coatingas part of a multi-functional shell via low temperature atomic layerdeposition (ALD) for sulfur-containing particles will be described. InALD, a chemical reaction proceeds on the surface of thesulfur-containing particles to form one shell layer at a time. For eachlayer, one gas precursor is introduced and chemisorbed on the surface ofthe sulfur-containing particles. The excess gas is flushed away andanother precursor gas is introduced to react with the first chemisorbedlayer, creating an additional monolayer of deposited film. Since the ALDprocess can be performed at very low temperatures, post-treatments(e.g., annealing of produced electrodes) can be used for additionalcontrol over the produced oxide microstructure.

An example process flow for the formation of a Li-ion conductive V₂O₅layer by ALD may include: 2VCl₄ (g)+5H₂O (adsorbed vapor)→V₂O₅ (s)+8HCl(g)+H₂ (g), or 2VO(C₃H₇)₃ (g)+3H₂O→V₂O₅ (s)+6HOC₃H₇ (g). Temperature,cycle time, and gas flow rate parameters can be tuned to improve theefficiency of the deposition conditions.

In a sixth example, a method of forming a metal oxide protective coatingas part of a multi-functional shell via high temperature chemical vapordeposition (CVD) for metal-sulfide containing particles will bedescribed. Deposition of oxides on any substrate can be challenging ifreactive metals (e.g., Li) are used, because most organometallicprecursors are very air- and moisture-sensitive, pyrophoric, andtherefore must be handled with extreme care. In addition, due to limitedshelf life span, precursors should be freshly synthesized prior tousage.

An example process flow for the formation of a Li-ion conductivealuminum oxide layer by CVD may include thermal decomposition of Altri-isopropoxide at above around 220° C.: 2C₉H₂₁O₃Al (vapor)→Al₂O₃(solid)+6 C₃H₇ (g)+C₃H₇O₃(g). The organometallic precursor (such as Altri-isopropoxide) vapors can be carried to the reaction site by acarrier gas, such as argon (Ar), helium (He) or even nitrogen (N₂).Temperature and gas flow rate parameters can be tuned to improve theefficiency of the deposition conditions.

The use of plasma enhancements for CVD or ALD processes may beadvantageous in some applications. For example, plasma enhancements mayallow the deposition temperature to be reduced while improving thecoating microstructure and uniformity. Due to the low evaporationtemperature of sulfur, plasma-enhanced (PE) PECVD and PEALD processesmay allow formation of ceramic coatings (shells) at temperatures wherethe vapor pressure of sulfur-based core particles is sufficiently small,such as at 100° C. or even at room temperature.

The materials and deposition process used should accordingly becarefully selected. For example, the most promising organometallicprecursors for ALD and CVD deposition processes should be identified(e.g., those that decompose at sufficiently low temperature, provideconstant vapor pressure, and produce the highest coating quality atlowest cost) from among the many types of commercially availableorganometallic precursors. Lithium precursors include Li β-diketonates(freshly synthesized and sublimed), Alky-Li (such as tert-ButylLithium,stripped from a solvent), and Lithium alcoholates (such as Li-Ethanolateor Li-hexafluoroisopropoxide, freshly synthesized). Common volatileprecursors for Mn (and other metals, such as V, Co, Ni, Al, etc.)include β-diketonates or alcoholates. These precursors have beensuccessfully used for CVD of high temperature super conductors and fuelcell materials.

Different gases can be used for the deposition process, including dryoxygen (O₂) as a reaction gas, and argon (Ar), helium (He) or evennitrogen (N₂) as a carrier gas. Both hot-wall and cold-wall reactors canbe used, with separate heated or cooled vaporizers to control thestoichiometry of the obtained film. Liquid delivery and/or flashvaporizers can further facilitate precursor handling and the overalldeposition process. Finally, deposition conditions should be selected insuch a way that reactions producing metal oxides (includinglithium-containing metal oxides) primarily occur on the surface of themetal-sulfide containing nanoparticles.

In a seventh example, a method of forming a metal oxide protectivecoating as part of a multi-functional shell via electroless depositionand solution precipitation will be described. Metal oxide coatings canbe formed via a solution precipitation method. For example, a Mn²⁺ saltprecursor may be added to a stable suspension of sulfur nanoparticles(or porous aggregates of sulfur nanoparticles). Over time, Mn²⁺ cationsare adsorbed on the sulfur surface, forming heterogeneous nucleationsites for MnO₂ shell growth. Permanganate MnO₄ ⁻ may then be added tooxidize the Mn²⁺ and produce a MnO₂ coating on the sulfur surface. Ahigh concentration of nucleation sites should be achieved prior toadding permanganate MnO₄ ⁻ to produce a conformal, defect-free MnO₂coating. Due to the solubility of metal-sulfides (e.g., lithium sulfide)in water, methods that use water as a solvent or as a processing gasshould be restricted to deposit oxide coatings on the surface of sulfur.

In an eighth example, a method of forming a metal oxide protectivecoating as part of a multi-functional shell via electrochemicaldeposition will be described. Powders and thin films of metal oxidematerials can be prepared by cathodic electrosynthesis from metal saltsolutions. In this method, metal ions or complexes are hydrolyzed by anelectrogenerated base to form oxide or hydroxide deposits (which can beconverted to corresponding oxides by thermal treatment) on cathodicsubstrates. Electrosynthesis of an organic phase and electrophoreticdeposition of charged polymers may then be used for coating formation.

In a ninth example, a method of forming a carbon coating as part of amulti-functional shell via low temperature solution based processes willbe described. The carbon coating can be formed on the surface of thesulfur-/metal-sulfide-containing core, or on the surface of othercoatings around the core (e.g., oxide, oxy-fluoride, fluoride,phosphate, iodide and other coatings).

Here, conductive carbon particles (such as graphene) and sulfurparticles (or sulfur particles coated with a layer of oxides, fluorides,oxyfluorides, iodides, phosphates, polymers or surfactants) may beprepared in such a way as to have opposite charge. For example, graphenecan be prepared to have a negative charge on its surface while sulfur orcoated sulfur particles can be prepared to have a positive charge ontheir surface. Mixing the suspension of positively charged sulfur orcoated sulfur particles with negatively charged carbon particles inducesthe formation of a conductive carbon coating around the sulfur or coatedsulfur particles.

The following is an example process flow for the formation of a carboncoating on the surface of a metal oxide (e.g., MnO₂) shell pre-depositedon the sulfur-containing core, and in particular, the synthesis ofC—MnO₂—S core-shell nanoparticles. In this process, highly flexible andhighly conductive graphene layers may be employed.

The process begins by preparing negatively charged graphene oxide (GO)via exfoliation of a natural graphite followed by ultrasonic shearing.While mechanical shearing may produce graphene of very high quality,ultrasonic treatment enables more scalable graphene production withsignificantly higher yield. Modifying negatively charged —OH functionalgroups on the MnO₂ shell surface may then be performed by surfacegrafting (e.g., of aminopropyltrimethoxysilane) to facilitate coating bypositively charged —NH₂ groups. Mixing the suspensions of negativelycharged graphene oxide particles and positively charged MnO₂—S particlesto induce co-assembling from mutual electrostatic interactions resultsin the formation of a stable suspension of negatively charged GO-MnO₂—Sparticles with an ultra-thin GO coating layer. Subsequent chemicalreduction of the GO with hydrazine results in graphene (carbon)encapsulated MnO₂—S particles. Centrifugation of the suspension may beused to isolate the produced C—MnO2-S particles.

In a tenth example, a method of forming a carbon coating as part of amulti-functional shell via low temperature hydrothermal decomposition oforganics in the presence of catalysts will be described. A method ofthis type may include the following three steps. First, the addition ofa carbon precursor solution (e.g., sucrose (C₁₂H₂₂O₁₁)) to a suspensionof sulfur nanoparticles may be performed to induce a sucrose coating ofthe sulfur nanoparticles. Second, hydrothermal carbonization may beperformed via dehydration of the sucrose and subsequent carbonization bythe addition of a catalyst (e.g., H₂SO₄). This process can take place atroom temperature, but elevated temperatures or pressures (through theuse of autoclave) can dramatically improve process kinetics. Third,centrifugation of the suspension may be performed to isolate theproduced C—S particles. If metal-sulfide nanoparticles are used insteadof sulfur, a water solution should not be used. Instead, a solution thatis a non-solvent for metal-sulfides but a solvent for the selectedorganic molecules should be used.

A similar process can be used to coat metal-sulfide (e.g., Li₂S) orcoated metal-sulfide (or sulfur or coated sulfur) particles with apolymer that is subsequently carbonized (transformation of a polymerinto an electrically conductive carbon layer). For example,metal-sulfide or coated metal-sulfide (or sulfur or coated sulfur)nanoparticles may be dispersed in an organic solvent, e.g.tetrahydrofuran (THF), which acts as a non-solvent for Li₂S whenpolycarbonate is the polymer. Polymer is then added and dissolved in themixture, with the ratio between the metal-sulfide or coatedmetal-sulfide (or sulfur or coated sulfur) nanoparticles and polymermeasured to control the thickness of the final polymer coating. Polymernon-solvent (e.g., ethanol, which is a non-solvent for polycarbonate) isslowly added to precipitate the polymer onto the nanoparticles. Thepolymer-coated nanoparticles may be separated via sedimentation,centrifugation or filtration, and then a carbon shell may be formed byannealing the polymer coated nanoparticles at high temperatures.

In order to induce more uniform coverage of the sulfur (or coatedsulfur, metal-sulfide, coated metal-Sulfide) particles with apolymer/organic layer, the particles and the polymer/organic moleculesshould have the opposite charge. For example, a positively chargedsurfactant-coated sulfur surface may be used with organic molecules thathave a negative charge on their surface (e.g., polysaccharides, such aspolyacrilic acid (PAA)).

In an eleventh example, a method of forming a carbon coating as part ofa multi-functional shell via chemical vapor deposition (CVD) will bedescribed. This process is more suited to metal-sulfides because itrequires temperatures in excess of 350° C. This process involves thermaldecomposition of organic precursors: C_(x)(H₂)_(y)(g)→yH₂(g)+xC(s). Bycontrolling the precursor flow rate, the degree of precursor dilution ininert gas, the pressure and temperature profile in the depositionchamber, and the nature of the precursor, the mean free path ofprecursor diffusion into the pores can be controlled before depositionoccurs. Under process conditions that limit carbon formation by asurface reaction rate rather than by precursor diffusion, uniform filmdeposition may be achieved. Doped carbon films can be produced by usinga carbon precursor containing elements other than C and H. In somecases, however, the CVD of carbon may not be appropriate. For example,the produced H₂ gas may react with a metal sulfide producing hydrogensulfides. To avoid such reactions, metal sulfide particles may be firstcoated with a conformal layer of metal oxides, such as aluminum oxide.Such coatings may be deposited by sol-gel, ALD, CVD, or other knownmethods.

In the twelfth example, a method of forming spherical porouscarbon-sulfur or carbon-sulfide core composites with a diameter in therange of about 100 nm to 80 microns and pores of about 1-100 nm in sizeis described. First, a porous carbon scaffold is produced. A mixture ofa monomer, optional porogen, and initiator in an organic solvent isdispersed in continuous phase (such as water or aqueous solutioncontaining surfactants and polymer stabilizers) to form sphericaldroplets of 200 nm-80 microns in diameter. By changing a dispersionroute and optimizing organic solvent/surfactant/monomer/polymerstabilizer combination, the size of the droplets may be tuned to achievethe desired average size. By the action of UV light or heat,near-spherical porous polymer particles may be produced.

After oxidation (e.g., by treatment in air or oxygen at around 200-300°C.) the polymer particles may be further stabilized. By thermaltreatment of the produced polymer particles in an inert environment(such as Ar or N₂ or He) or in a mild oxidizing atmosphere (such as CO₂)spherical porous carbon particles are produced. The porogen content andchemistry in the dispersed phase will impact the size of the large-sizepores (e.g., about 2-200 nm). An optional activation procedure (e.g., byexposing the produced carbon powder samples into the stream of CO₂ gasor steam at around 800-1000° C.) is then used to increase the porevolume and form smaller pores (e.g., about 0.5-5 nm) within the porouscarbon particles.

Second, the electrically conductive porous carbon scaffold isinfiltrated with sulfur or metal sulfides. In one example, sulfur (ormetal sulfide) can be melt-infiltrated into the porous carbon scaffoldby immersion of carbon particles into a sulfur (or metal sulfide) melt.The excess of sulfur (or a metal sulfide) from the particle surface canbe removed by evaporation. The pores within the C—S composite core cansimilarly be formed by partial evaporation of sulfur. Alternatively,sulfur (or metal sulfide) can be introduced by dissolving it in asolvent, infiltrating the porous carbon scaffold by a sulfur- (orsulfide-) containing solution and evaporating the solvent. Therepetition of this procedure multiple times may allow filling themajority of pores in carbon.

In yet another scenario, a non-solvent for sulfur (or a metal sulfide)such as acetone can be induced into the core of the porous carbonscaffold particles and induce nucleation of sulfur (or metal sulfide)particles upon the porous carbon scaffold particle emersion into thesulfur (or metal sulfide) solution. The excess of sulfur (or metalsulfide) solution can be removed by dipping the particles into a solventand drying. The repetition of this procedure will similarly allowfilling the majority of pores in carbon with sulfur or metal sulfides.Third, the external surfaces of the carbon-sulfur (or carbon-sulfide)particles are coated with a metal ion permeable (but polysulfideimpermeable) shell.

In a thirteenth example, a method of forming a composite carbon-ceramicor carbon-polymer shell is described. First, a porous carbon shell isproduced by, for example, formation of a polymer coating and itssubsequent decomposition into a porous carbon layer. The use of porogenor activation may allow one to increase the volume of the pore in thecarbon shell. Alternatively, evaporation (or dissolution) of sulfur (ormetal sulfide) from the surface layer of the S—C(or metalsulfide-carbon) composite previously described in the twelfth exampleabove may similarly allow for the formation of a porous carbon shell.Once the porous carbon shell is formed, it can then be infiltrated witha polymer or ceramic filler (impermeable to metal sulfide but ideallypermeable to metal ions). In the case of a ceramic filler, CVD, ALD,sol-gel, and other methods (including the ones previously described) maybe utilized.

FIG. 12 shows example electrochemical performance data of an electrodeproduced according an example embodiment as compared to performance of aconventional electrode. For this example, composites composed of aporous carbon-sulfur core were suspended in an aqueous solution of apolyacrilic acid (PAA) binder, cast on a metal foil, and dried. Afterdrying, the electrode was divided into several portions. One portion wasdirectly used as a cathode for the formation of a cell with a lithiumfoil anode (99.9% purity). The test cells were assembled inside an argonglovebox (less than 1 ppm of H₂O). The electrolyte was composed of a 3Mbis(triflouromethanesulfonyl)imide (LiTFSI) salt dissolved in a mixtureof dimethoxyethane (DME):1,3-dioxane (DIOX) solvents as electrolyte. 0.2M LiNO₃ (Alfa Aesar, 99.99%) was added to the electrolyte as anelectrolyte additive. The charge-discharge tests were conducted at 70°C., between 3.0 and 1.2 V vs. Li/Li⁺ in a galvanostatic mode. As shown,the conventional cell shows less than 150 stable cycles (top graph).Another portion of the electrode was additionally coated with aconformal layer of aluminum oxide (around ˜5 nm) by using aplasma-enhanced atomic layer deposition (ALD) technique. After the shellformation by ALD, the cell was assembled with the alumina coatedelectrode in exactly the same way as for the conventional electrode. Thesame electrolyte composition and testing regime was used. However, asfurther shown, more than 350 stable cycles were achieved after the shellcoating step was implemented (bottom graph).

FIG. 13 illustrates an example Li-ion battery in which the abovedevices, methods, and other techniques, or combinations thereof, may beapplied according to various embodiments. A cylindrical battery is shownhere for illustration purposes, but other types of arrangements,including prismatic or pouch (laminate-type) batteries, may also be usedas desired. The example Li-ion battery 1301 includes a negative anode1302, a positive cathode 1303, a separator 1304 interposed between theanode 1302 and the cathode 1303, an electrolyte (not shown) impregnatingthe separator 1304, a battery case 1305, and a sealing member 1306sealing the battery case 1305. It will be appreciated that the exampleLi-ion battery 1301 may simultaneously embody multiple aspects of thepresent invention in various designs.

The preceding description is provided to enable any person skilled inthe art to make or use embodiments of the present invention. It will beappreciated, however, that the present invention is not limited to theparticular formulations, process steps, and materials disclosed herein,as various modifications to these embodiments will be readily apparentto those skilled in the art. That is, the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention, which should only be defined by thefollowing claims and all equivalents.

The invention claimed is:
 1. A battery electrode composition comprising core-shell composites, each of the core-shell composites comprising: a metal sulfide-based core provided to electrochemically react with metal ions during battery operation to store the metal ions in the form of a corresponding metal-sulfide during discharging of the battery and to release the metal ions from the corresponding metal-sulfide during charging of the battery; and a multi-functional shell at least partially encasing the metal sulfide-based core, the multi-functional shell being formed from a material that has an average diffusivity that is higher by a factor of at least 100 with respect to (i) the metal ions of the metal sulfide as compared to (ii) electrolyte solvent molecules and metal polysulfides.
 2. The battery electrode composition of claim 1, wherein the metal sulfide is a lithium sulfide and the metal ions that are stored and released are Li⁺cations.
 3. The battery electrode composition of claim 1, wherein the metal sulfide is a sodium sulfide and the metal ions that are stored and released are Na⁺cations.
 4. The battery electrode composition of claim 1, wherein the metal sulfide-based core is a dense agglomerate of individual metal sulfide particles, each of which is at least partially encased in a metal-ion permeable shell.
 5. The battery electrode composition of claim 1, wherein the metal sulfide-based core comprises electrically conductive carbon for structurally reinforcing the metal sulfide-based core against changes in volume.
 6. The battery electrode composition of claim 5, wherein the electrically conductive carbon comprises disordered carbon.
 7. The battery electrode composition of claim 5, wherein the electrically conductive carbon comprises carbon associated with thermal decomposition of a polymeric precursor.
 8. The battery electrode composition of claim 1, wherein the metal sulfide-based core corresponds to a product of a solution of the metal sulfide.
 9. The battery electrode composition of claim 1, wherein the multi-functional shell comprises carbon.
 10. The battery electrode composition of claim 1, wherein the multi-functional shell, the metal sulfide-based core, or both the multi-functional shell and the metal sulfide-based core comprise carbon.
 11. The battery electrode composition of claim 10, wherein the carbon corresponds to a thermal decomposition product derived from a polymeric coating deposited on a surface of the metal sulfide in a solution.
 12. The battery electrode composition of claim 11, wherein the polymeric coating has an opposite electrical charge as compared to the metal sulfide in the solution.
 13. The battery electrode composition of claim 10, wherein at least one type of carbon in the multi-functional shell, the metal sulfide-based core, or both corresponds to a thermal decomposition product derived from a gaseous hydrocarbon precursor and chemical vapor deposition.
 14. The battery electrode composition of claim 10, wherein at least one type of carbon in the multi-functional shell, the metal sulfide-based core, or both is porous and is infiltrated with a different filler material.
 15. The battery electrode composition of claim 14, wherein the filler material is a carbon-based material different from the carbon in the corresponding multi-functional shell or the metal sulfide-based core.
 16. The battery electrode composition of claim 14, wherein the filler material is a metal ion-permeable ceramic material.
 17. A process of fabricating the battery electrode composition of claim 1, comprising annealing the core-shell composites at a temperature below the melting point of the metal sulfide.
 18. A process of fabricating the battery electrode composition of claim 1, comprising introducing one or more additives into a solution of the metal sulfide to induce formation of a core-supporting skeleton.
 19. The process of claim 18, wherein the additives are selected from the group consisting of: carbon particles, ceramic particles, and a polymer.
 20. A process of fabricating the battery electrode composition of claim 1, comprising formation of one or more miniemulsions.
 21. The battery electrode composition of claim 1, wherein the average diffusivity of the material is higher by a factor of at least 1000 with respect to (i) the metal ions of the metal sulfide as compared to (ii) electrolyte solvent molecules and metal polysulfides.
 22. The battery electrode composition of 21, wherein the average diffusivity of the material is higher by a factor of at least 10000 with respect to (i) the metal ions of the metal sulfide as compared to (ii) electrolyte solvent molecules and metal polysulfides. 